Semiconductor materials for a long time were synthesized by different epitaxial methods. There are several known epitaxial growth techniques such as liquid phase epitaxy (LPE), chloride vapor epitaxy (ClVPE), hydride vapor epitaxy (HVPE), molecular beam epitaxy (MBE), chemical beam epitaxy (CBE) and metal organic phase epitaxy (MOVPE). Details concerning MBE method are shown e.g. in work of M. A. Herman (M. A. Herman, H. Sitter “Molecular Beam Epitaxy, Fundamentals and Current Status” in Springer Series in Materials Science, 2nd Edition Springer-Verlag Berlin Heidelberg New York 1996, ISBN 3-540-60594-0).
Nitrogen exists as a two-atom molecules which binding energy is so high that nitrogen molecules can not decompose into single atoms (necessary for epitaxial growth) when they are in contact with hot substrate. Due to this fact, in nitride epitaxial process two methods of generation of active nitrogen atoms are used. In the first method atomic nitrogen is created as a result of ammonia (NH3) decomposition (decomposition is possible due to the high temperature on grown substrate) but in parallel, atomic hydrogen is created. In second method, decomposition or excitation of nitrogen molecules takes place before supplying it to the surface.
Up to now, two methods of making semiconductor laser diodes based on gallium nitride, indium nitride, aluminum nitride and their alloys by epitaxial growth have been known—MOVPE and Gas Source MBE (GSMBE).
The MOVPE method, relies on making use of MOVPE epitaxy with ammonia (NH3) described for instance in work of S. Nakamura (Shuji Nakamura, Gerhard Fasol “The Blue Laser Diode, GaN Based Emitters and Lasers” Springer-Verlag 1997 (ISBN 3-540-61590-3)). Using this method, in 1995 the first, pulsed operated, blue laser diode was demonstrated. Shortly after the continuous wave lasing was obtained in 1999, violet laser diodes with optical power of 5 mW were offered on the marked by Nichija company. Up until now, laser action was demonstrated in only few research groups (e.g three in USA, three in Europe including Institute of High Pressure PAS as well as one in Korea and one in Taiwan). Design and way of fabrication of a blue-violet semiconductor laser diode is described e.g. in the cited already work of S. Nakamura and G. Fasol (Shuji Nakamura, Gerhard Fasol “The Blue Laser Diode, GaN Based Emitters and Lasers” Springer-Verlag 1997 (ISBN 3-540-61590-3)). Example design of such diode consists of a sapphire substrate on which the following layers are grown: 30 nm GaN buffer, 3 μm GaN:Si, 0.1 μm In0.05Ga0.95N:Si, 0.5 μm Al0.07Ga0.93N:Si, 0.1 μm GaN:Si, three quantum wells (5 nm In0.14Ga0.86N wells, 10 nm In0.02Ga0.98N barriers), 20 nm Al0.2Ga0.8N:Mg, 0.1 μm GaN:Mg, 0.5 μm Al0.07Ga0.93N:Mg, 0.2 μm GaN:Mg. The p-type doping is reached by introduction into reactor chamber Cp2Mg compound.
The GSMBE method relies on epitaxial process where group V elements are supplied by gas sources. GSMBE of nitrides, similarly to MOVPE method uses NH3 is a gas which delivers nitrogen to the layer and Cp2Mg as a source of p-type doping. Details of this method is shown e.g. in work by (S. E. Hooper, M. Kauer, V. Bousquet, K. Johnson, J. M. Barnes and J. Heffernan, Electronic Letters vol. 40, 8th Jan. 2004).
Thus, a common feature of the both methods is use of ammonia as a source of nitrogen in synthesized layers. During decomposition of ammonia on the hot surface, large quantity of hydrogen is released, which enters into the grown crystal and may deteriorate the layers quality. For instance, hydrogen compensates magnesium acceptors (bond H—Mg is created) and special thermal procedure is required after the growth process to activate p-type conductivity as shown earlier in the work of S. Nakamura and G. Fasol. Although authors of the GSMBE method claim that there is no need for thermal activation of the layers after the growth process in order to achieve p-type conductivity, the inferior parameters of laser diodes produced by the GSMBE may be due to the difficulties in Mg activation in the layers far away from the surface in the vicinity of the quantum wells. In general, it is worth to note that hydrogen which is present in high concentration in layers grown with NH3 may degrade quality and change parameters of layers grown by MOVPE and GSMBE. This can impact negatively properties such as stability of InGaN quantum wells or raise the values of threshold current density.
In epitaxial growth technology a molecular beam epitaxy method is also known, called Reactive Nitrogen MBE (RN-MBE), which uses chemically active atoms and molecules of nitrogen. One of the versions of the RN-MBE is Plasma Assisted MBE (PAMBE). PAMBE relies on the fact that constituent layer atoms such as Ga, In, Al, Si, Mg are supplied by evaporation of these species from effusion cells (where flux of given components is a function of the temperature of effusion cell). The active nitrogen in the PA MBE is supplied by the plasma unit in a form of a beam of excited molecules and atoms. The plasma unit is a device which excites (or dissociates) nitrogen molecules (in order to create nitrides of gallium, indium, aluminum and their alloys). For instance, it may be a device where radio-frequency radiation (RF plasma source) or cyclotron resonance effect (ECR plasma source) excites nitrogen molecules.